Stud welding method and stud welding device, especially for stud welding without a support foot

ABSTRACT

According to a first embodiment of the method, the stud holder is first positioned without a part to be welded at a certain distance from the tool surface. A part to be welded is then provided. When the welding axle moves towards the workpiece, the path that needs to be travelled in order to bring the part to be welded and the workpiece into contact is detected. Conclusions regarding the presence or absence of a part to be welded are then drawn, based on the previous distance between the stud holder and the workpiece and the path travelled. According to another embodiment, the stud holder is moved towards the workpiece with a part to be welded until contact is detected. If, after contact is detected, further movement of the welding axle is possible as a result of the part to be welded being pushed into the stud holder, this is interpreted as an indication of the presence of a part to be welded.

[0001] The invention relates to a method and a device for stud welding,particularly for stud welding without a supporting foot.

[0002] In stud welding devices predominantly used up to now, a weldinghead or, for manual welding, a welding gun is employed, with asupporting foot or a spacer being provided at the front end of thewelding head or the welding gun, respectively. Thus, after placing thestud welding head or the welding gun upon the surface of the workpiecewith the supporting foot, a defined position is obtained, particularly adefined distance between the welding head or the welding gun and thesurface of the workpiece. Then, taking this defined position as astarting position, the stud holding device or chuck and the part heldtherein and to be welded can be moved relative to the surface of theworkpiece.

[0003] A stud welding device of this kind is described in the documentDE 32 15 453 C1, for example. In this stud welding device, the studwelding gun comprises a drive in the form of an electric motor for thewelding axle, with a d.c. motor driving the welding axle (here, themoving part assembly, which comprises all moving parts that cooperate toperform the actual welding movement such as the charging head, the feedcylinder, the feed piston, etc.) and the stud holding device connectedtherewith in the axial direction. After the welding gun has been placedupon the surface of the workpiece with the supporting foot, the studholding device is first moved towards the surface of the workpiece untilthe face of the part held in the stud holding device and to be weldedcontacts the surface of the workpiece. Then, the current for thepre-current arc or pilot arc is switched on and the welding axle islifted off from the surface of the workpiece by means of the d.c. motor.Hereby, a pre-current arc is struck. The further course corresponds tothe usual process of stud welding with stroke ignition where a pilot arcis struck by placing a stud in good electrical contact with theworkpiece, switching on the pre-current and retracting the stud from theworkpiece.

[0004] Furthermore, from the document EP 0 776 261 B1, a device forwelding together at least two parts is known, in which a welding head isoriented opposite to the intended welding area of the workpiece and isthen moved towards the workpiece by means of an adjustment drive, e.g.the arm of a robot, until the face of the part to be welded is pressedonto the surface of the workpiece. In the course of this procedure, thewelding head is moved towards the workpiece beyond the position in whichthe part to be welded touches the surface of the workpiece for the firsttime. For this purpose, the welding axle of the welding head isconfigured such that it is moved into the welding head when the weldinghead is moved further towards the workpiece in this way. By providingsuch an additional path of movement for the welding head, it is possibleto make steady contact between the face of the part to be welded and theworkpiece within a large range independently of positional tolerances ofthe workpiece and of tolerances occurring during the movement of thewelding head. After the movement of the welding head has been stopped,the welding axle of the welding head can be moved together with the studholding device and the part to be welded held therein, starting from thestarting position thus created, in order to continue the weldingoperation. This movement is effected by means of the drive for thewelding axle. Thus, this device and this method make welding without asupporting foot possible.

[0005] Welding without a supporting foot has the advantage, amongothers, that the welding area, i.e. the area of the surface of theworkpiece which has to be accessible for welding, may be markedlysmaller compared to welding with a supporting foot. In particular, whenwelding with a supporting foot, it is hardly possible to weld a part tobe welded to a workpiece in relatively narrow grooves of this workpiece.

[0006] Particularly in stud welding methods or stud welding devicesworking without a supporting foot, however, there is a certain dangerthat if there is no part to be welded in the stud holding device, thestud holding device itself will be welded to the workpiece.

[0007] In stud welding devices working with a supporting foot, thepresence of a part to be welded in the stud holding device could easilybe detected by measuring after placing the welding head upon theworkpiece—the distance which the welding axle has to cover when it ismoved towards the workpiece until a contact is made. Then, knowing thegeometry of the part to be welded and its position in the stud holdingdevice, one can draw conclusions on the presence of a part to be welded,if the distance covered by the welding axle until the part to be weldedcontacts the workpiece is smaller than a predetermined distance. Thismethod, however, fails in case of stud welding devices working without asupporting foot.

[0008] Accordingly, the object of the invention is to provide a studwelding method and a stud welding device, particularly for stud weldingwithout a supporting foot, in which the presence of a part to be weldedin the stud holding device is securely detectable.

[0009] The invention achieves this object with the features of patentclaims 1 and 10 and 15. Furthermore, the invention provides a program,particularly for upgrading existing stud welding devices, in order to beable to perform the method according to the invention.

[0010] According to an embodiment of the invention, the stud holdingdevice is at first moved towards the surface of the workpiece without apart to be welded being held therein, at least until the face of thepart contacts the surface of the workpiece. Then, the stud holdingdevice is lifted off from the workpiece until a predetermined distancebetween the face of the stud holding device and the surface of theworkpiece is reached. Afterwards, or partially at the same time, a partto be welded can be supplied to the stud holding device. This ispreferably done by means of a customary automatic stud supply device.Supplying the stud is effected in such a way that the face of the partto be welded projects from the face of the stud holding device by apredetermined amount.

[0011] Then, the stud holding device is moved towards the surface of theworkpiece again until a contact between the face of the part to bewelded and the surface of the workpiece is detected. The distance thatis covered starting from the distance adjusted primarily between thestud holding device and the workpiece until the contact is detected,allows to draw the conclusion as to whether a part to be welded is heldin the stud holding device or not. If the distance covered that has beendetected basically corresponds to the distance between the face of thestud holding device and the surface of the workpiece adjusted before, itmay be concluded with a high degree of certainty that no part to bewelded is held in the stud holding device. However, if the distancecovered is smaller than the distance adjusted before, it may beconcluded that there a part to be welded is present in the stud holdingdevice. In this case, the welding process can be carried out. Otherwise,the welding process can be blocked or measures can be taken, such aslifting off the stud holding device from the workpiece again by asufficient amount and supplying a part to be welded again.

[0012] On this occasion, it may be remarked that the movement of thestud holding device may be effected either by a movement of the entirewelding head, e.g. by an arm of a robot or the like, or by a movement ofthe welding axle only by means of a drive provided in the welding head.

[0013] The method explained above also makes it possible to drawconclusions on an admissible position of the part to be welded in thestud holding device. It is again the difference between thepredetermined distance and the path detected which is covered aftersupplying the part to be welded, starting from the predetermineddistance, until the point of contact is reached, which may serve thispurpose. If this path is within a predetermined range of admissibility,conclusions may be drawn on a correct axial position of the part to bewelded in the stud holding device, particularly on a sufficient lengthof projection.

[0014] According to an embodiment of the invention, the first movementof the stud holding device towards the surface of the workpiece can onlybe effected until a (first) contact between the stud holding device andthe workpiece is detected.

[0015] Detecting the contact between the stud holding device and thesurface of the workpiece or between the part to be welded and thesurface of the workpiece may quite generally be effected e.g. bydetecting an electric or mechanical contact.

[0016] According to an embodiment of the invention, effecting themovements for the stud holding device in the way described above mayexclusively be done by a drive for the stud holding device provided inthe welding head of the stud holding device or by an element coupledtherewith.

[0017] In this embodiment, another drive for positioning the entirewelding head will usually be provided, with which the welding head isfirst positioned relative to the workpiece in such a way that a distanceremains between the face of the stud holding device and the surface ofthe workpiece at the desired welding area. Then, starting from thisposition, the movements described above can be effected and the measuresfor detecting the presence of a part to be welded in the stud holdingdevice described above can be taken.

[0018] In another embodiment of the invention, moving the stud holdingdevice until a contact between the face of the stud holding device andthe surface of the workpiece is achieved can also be effected by meansof a positioning drive for the entire stud welding head. Here,corresponding to the method and the device described in EP 0 776 261 B1,for reasons of simplicity, the movement of the stud holding device bymeans of the positioning drive for the stud welding head will beeffected in such a way that a contact between the stud holding deviceand the surface of the workpiece is achieved with a sufficient degree ofcertainty even if relatively large tolerances for the position of theworkpiece or the positioning of the welding head or the position of thepart to be welded in the stud holding device may occur. For thispurpose, the positioning drive may e.g. be provided with one or morevalues defining an absolute position in space, which position can bereached by means of open or closed loop control.

[0019] Thus, with this method, the stud welding head is usually movedtowards the workpiece beyond the point of first contact between the studholding device and the workpiece. In this case, the stud holding deviceor the drive for the stud holding device (or the welding axle) has to beconfigured such that the stud holding device can be moved into thewelding head in case of an (excess) movement of the welding head towardsthe workpiece.

[0020] Starting from the starting position reached then, the furthermovements can be made and the further measures for detecting thepresence of a part to be welded in the workpiece as explained above canbe taken.

[0021] In another embodiment of the invention, a part to be welded isalready held in the stud holding device when the stud holding device ismoved towards the workpiece for the first time. Here, the stud holdingdevice is again moved towards the surface of the workpiece until theface of the part to be welded contacts the surface of the workpiece. Forthis purpose, the part to be welded is held in the stud holdingdevice—preferably in a locked position—in such a way that the face ofthe part to be welded projects from the face of the stud holding deviceby a predetermined length of projection. This position of the part to bewelded corresponds to the usual position in which welding is possible.

[0022] After detecting a contact between the part to be welded and theworkpiece, the locked position of the part to be welded in the studholding device is released so that an axial movement of the part to bewelded relative to the stud holding device is possible. After releasingthe locked position, an attempt is made to move the stud holding devicefurther towards the workpiece. A possible further movement is detected.Instead of holding the part to be welded in a locked position, the partcan also be held in such a way that the part to be welded is not pushedinto the stud holding device before a predetermined force of touch-down(of the part on the workpiece) is exceeded.

[0023] If a further movement towards the workpiece is detected, one candraw conclusions on the presence of a part in the stud holding device.

[0024] If no further movement can be detected, it is assumed that thereis no part to be welded in the stud holding device and the weldingprocess is stopped or further measures for supplying a part to be weldedto the stud holding device are taken.

[0025] If, however, the presence of a part to be welded has beendetected, the stud holding device will be lifted off from the surface ofthe workpiece by a sufficient amount and the part to be welded is movedto the starting position (welding position) again. Then, the weldingprocess can be carried out in the usual way.

[0026] According to an embodiment of this method of the invention, theattempt of further movement can be stopped when the power consumption ofa drive for the stud holding device and the welding head (depending onwhich drive is to be used for making the further movement) or the forceacting during the attempt of movement in the stud holding device or inan element coupled therewith exceeds a predetermined value.

[0027] If no movement of the stud holding device exceeding apredetermined value is detected between the beginning of the attempt ofmovement and the moment when this attempt is stopped, the conclusion isdrawn that a part to be welded is missing in the stud holding device.

[0028] In this embodiment of the method according to the invention,after the part to be welded has been moved into its starting position inthe stud holding device again, the stud holding device can bere-positioned regarding its distance to the surface of the workpiece orcan be placed onto the surface of the workpiece again. As alreadyexplained, this may be done by moving the stud holding device towardsthe workpiece until a contact is detected and, as the case may be, bysubsequently lifting off the stud holding device by a predetermineddistance.

[0029] According to another embodiment, a potential contact with thesurface of the workpiece is monitored when supplying the part to bewelded, too. If a contact is already detected during the process ofsupplying, one may conclude with a high degree of certainty that a partto be welded was already held in the stud holding device before theprocess of supplying. In this case, the forward movement of the part tobe welded in the stud holding device can be stopped.

[0030] On this occasion, it may be remarked that a part to be weldedshould only be supplied to the stud holding device if there is asufficient distance between the face of the stud holding device and thesurface of the workpiece. This distance must in any case be larger thanthe length of projection which is to be given after the part to bewelded has been supplied.

[0031] If it is detected with the above-mentioned measure that a part tobe welded was already present in the stud holding device before theprocess of supplying, the stud holding device can be moved into aposition in which—as the case may be, by starting the supply movement ofthe charging pin again—the part to be welded that was already presentbefore can be ejected from the stud holding device.

[0032] The method according to the invention may be integrated inalready existing stud welding devices which comprise a microprocessorcontrol unit without any substantial changes in hardware. For thispurpose, the method according to the invention may be realized byproviding a control program (software) which can be integrated in analready existing evaluation and control unit.

[0033] Further embodiments of the invention will be apparent from thesubclaims.

[0034] In the following, the invention is explained in greater detailwith the aid of embodiments shown in the drawings, in which:

[0035]FIG. 1 is a schematic view showing a stud welding device accordingto the invention;

[0036]FIG. 2 is a schematic view showing different phases of themovement of the stud holding device for realizing a first embodiment ofthe method according to the invention; and

[0037]FIG. 3 is a schematic view showing different phases of themovement of the stud holding device for realizing a second embodiment ofthe method according to the invention.

[0038]FIG. 1 shows a stud welding device 1 comprising a stud weldinghead 3 which in turn consists of the actual welding axle 5 and a drive 7for axially moving the welding axle 5 (arrow I in FIG. 1).

[0039] As indicated in FIG. 1, the entire welding head 3 may be arrangedat the arm 9 of a robot by which the welding head 3 is movable in one orseveral axes.

[0040] Both the drive 7 for the welding axle 5 and a drive 11 for thearm 9 of a robot are controlled by an evaluation and control unit 13.

[0041] Here, the evaluation and control unit 13 can additionally takeover (in a way not illustrated) the further controlling of the weldinghead 3 regarding the control of the welding current. As the presentinvention basically relates to a movement of the welding head 3 or of astud holding device 15 of the welding axle 5, the illustration offurther components of the stud welding device 1 was omitted.

[0042]FIG. 1 also shows a workpiece 17 onto which a part (stud) 19 to bewelded, which is held in the stud holding device 15, is to be welded.The workpiece 17 is in electric contact with the evaluation and controlunit 13. This is required firstly for the flow of a welding currentduring the welding operation and secondly for detecting an electriccontact between the stud holding device 15 or the part 19 to be weldedand the workpiece 17 in the way explained below.

[0043]FIG. 1 furthermore shows that parts 19 to be welded may besupplied to the welding axle 5 of the stud welding head 3 by means of anautomatic supply device which is not illustrated in greater detail. Thisis done by means of a supply conduit 21 in the front portion of thewelding head 3, which is connected with the automatic supply device inthe usual way by means of a hose (not shown).

[0044] Supplying a part 19 to be welded is effected by means of blastair, whereby the part to be welded is moved to the front portion of thestud welding head 3 via the supply conduit 21. In this position, thepart to be welded already substantially lies in the axis of the weldingaxle 5 or the stud holding device 15. The part 19 to be welded is thenpushed through the stud holding device to the front portion thereof. bya charging pin 23 into an axial position in which welding of the part 19is possible. For this purpose, the part 19 of course has to project fromthe face of the stud by a predetermined projecting length d. Thecharging pin 23 is acted upon by a drive 25 which is only illustratedschematically in FIG. 1 and which is usually configured as a pneumaticdrive. The drive 25 makes it possible to move the charging pin 23between a front position, in which the face of the charging pin 23 actsupon the rear face of the part 19 to be welded, and a retracted positionin which the front portion of the charging pin 23 is retracted to suchan extent that the supply conduit 21 is free for supplying a part to bewelded. The drive 25 is controlled by the evaluation and control unit13, as well.

[0045] In the following, the stud welding method according to theinvention carried out with a device shown in FIG. 1 is explained ingreater detail.

[0046] For this purpose, FIG. 2 shows different phases or individualsteps of a welding operation, with only the front portion of the weldingaxle 5 being illustrated, respectively, as well as the workpiece 17 ontowhich one or several parts to be welded are to be welded.

[0047] The phase illustrated in FIG. 2a shows the welding axle 5,already positioned in relation to the workpiece 5. The position shown inFIG. 2a can either be reached by correspondingly controlling the drive 7for the welding axle 5 or by correspondingly controlling the drive 11for the arm 9 of a robot.

[0048] On this occasion, it may be remarked that controlling the drive11 for the arm 9 of the robot may not only be effected by the evaluationand control unit 13, as illustrated in FIG. 1, but of course also by anindividual evaluation and control unit for the arm of the robot. Forthis purpose, the two control units may of course communicate with eachother in order to guarantee that the welding process proceeds correctlyand as efficiently as possible.

[0049] In the position of the welding axle 5 shown in FIG. 2a, the faceof the stud holding device 15 is spaced from the surface of theworkpiece 17 by a distance X_(C). Taking this position as a startingpoint, the welding axle 5 is moved towards the surface of the workpieceuntil the face of the stud holding device 15 contacts the surface of theworkpiece. This phase is illustrated in FIG. 2b. Moving the welding axlemay be effected by one of the two drives 7 or 11. Preferably, however,this movement will be effected by the drive 7, as this drive usuallymakes a more exact positioning of the welding axle 5 in the axialdirection thereof possible.

[0050] Detecting a contact between the face of the stud holding device15 and the surface of the workpiece 17 may be effected by applying ameasuring voltage between the workpiece 17 and the stud holding device5. As soon as a flow of current is detected, one may assume that thereis an electric contact between the stud holding device and theworkpiece, and after the flow of current has been detected, the drive 7(or, as the case may be, the drive 11) is stopped.

[0051] However, a contact between the stud holding device 15 and theworkpiece 17 can also be detected e.g. by measuring the driving power ofthe respective drive, preferably drive 7. As soon as the driving powersuddenly increases drastically as a result of a mechanical contactbetween the workpiece and the stud holding device, it is assumed thatthere is mechanical contact, and the drive is stopped.

[0052] Such an evaluation for detecting an electric or mechanicalcontact may be carried out in the usual way by the evaluation andcontrol unit. As already mentioned before, for detecting an electriccontact, an electrically conductive connection between the workpiece andthe evaluation and control unit 13 is made. Furthermore, of course therehas to be an electrically conductive connection between the evaluationand control unit 13 and the stud holding device. These electricconnections, however, are necessary anyway, as the evaluation andcontrol unit 13 is usually also designed for supplying a weldingcurrent.

[0053] For detecting a mechanical contact, the evaluation and controlunit 13 can detect the power consumption of the drive 7 or the drive 11,too. For this purpose, suitable means may be provided in the evaluationand control unit 13. After placing the stud holding device onto thesurface as shown in FIG. 2, the evaluation and control unit 13 controlsthe drive 7 (alternatively, 11) in such a way that the face of the studholding device 15 is lifted off from the surface of the workpiece by adefined distance X_(c).

[0054] Then, a part 19 to be welded may be fed to the welding axle viathe supply conduit 21. For this purpose, the evaluation and control unit13 may output a signal to a control unit for an automatic supply device(not illustrated). After supplying the part 19 to be welded, this partis moved forward by means of the charging pin 23 in the stud holdingdevice 15 until the welding position is reached. This position or thisphase of the welding operation is illustrated in FIG. 2d.

[0055] Of course, care must be taken that the distance X_(c) between theface of the stud holding device 15 and the surface of the workpiece inFIG. 2c is larger or equal to the length d of projection by which theface of the part 19 to be welded projects from the face of the studholding device 15.

[0056] In the next step, the welding axle 5 is again moved towards thesurface of the workpiece until a contact between the face of the part 19to be welded and the surface of the workpiece is detected. As describedabove, this detection may again be effected by detecting an electric ormechanical contact.

[0057] For detecting a mechanical contact in particular, it is importantthat the charging pin 23 acts upon the rear side of the part 19 to bewelded in order to keep this part in the welding position even when itsface is placed upon the surface of the workpiece.

[0058] If it is not intended that it acts upon it continuously (which isprincipally possible), after placing the part onto the surface andlifting it off again, as described below, care has to be taken that thepart 19 to be welded is pushed forward again into the welding positionby moving the charging pin 23 again.

[0059] The evaluation and control unit 13 detects the distance (X_(c)−d)by which the welding axle 5 has to be moved at the transition point fromthe phase shown in FIG. 2d to the phase shown in FIG. 2e. If thedistance detected is smaller than the distance X_(c) adjusted before,the evaluation and control unit will assume that a part 19 to be weldedis present in the stud holding device 15.

[0060] However, if the evaluation and control unit 13 ascertains thatthe distance detected is substantially equal to the distance X_(c)adjusted before (or that the difference between the distance X_(c) andthe distance detected is smaller than a predetermined threshold), itwill assume that no part to be welded is present in the stud holdingdevice.

[0061] However, the difference between the distance X_(c) and thedistance traveled at the transition point between the phases shown inFIGS. 2d and 2 e may not only be interpreted or evaluated in the senseof the presence or absence of a part to be welded, but also with respectto a correct axial position of the part 19 to be welded in the studholding device 15. For example, it may be assumed that the position iscorrect if the difference between the distance X_(c) and the distancetraveled is within a predetermined range.

[0062] However, if it is presupposed that the stud was pushed into thecorrect position within the stud holding device 15 by means of thecharging pin 23, when using studs with different lengths one may drawconclusions on whether a stud having the desired length is present inthe stud holding device, because without any adjustment of the front endposition of the charging pin 23, a longer stud will project more fromthe front end of the stud holding device (i.e. a smaller distanceX_(c)−d is detected) than a shorter stud.

[0063] If the absence of a part 19 to be welded or an incorrect axialposition thereof in the stud holding device is detected, either an errorsignal or an operator recall signal may be generated, or measures forfailure corrective action may be taken. For example, if the absence of apart 19 to be welded is detected, another supply operation may betriggered.

[0064] If the presence and, as the case may be, the correct position ofa part 19 to be welded in the stud holding device 15 are detected, theactual welding operation may be triggered in the phase shown in FIG. 2e.For example, in the case of welding with stroke ignition, thepre-current may be switched on and then, as illustrated in FIG. 2f, thewelding axle 5 may be lifted off from the surface of the workpiece by apredetermined distance so that the pre-current arc is created.

[0065] Then, the main current electric arc may be started and thewelding axle may be moved towards the workpiece and may be immersed inthe molten mass after a predetermined welding time.

[0066] In practice, a case may occur in which, for certain reasons,another part to be welded is already held in the stud holding device 15before a part 19 to be welded is fed to the stud holding device.

[0067] In the process shown in FIG. 2, in the phase according to FIG.2d, this would have the consequence that, when supplying another part tobe welded, the part that is already present in the stud holding device15 would be moved further to the front, i.e. towards the surface of theworkpiece. If the length of the part 19 to be welded exceeds thedistance X_(c) adjusted before, this will lead to a contact between theface of the part to be welded that was already present before and thesurface of the workpiece. This contact may in turn be detected. If theevaluation and control unit 13 already detects an electric or mechanicalcontact during the charging operation, i.e. during the movement of thecharging pin 23 by means of the drive 25, this will be interpreted asthe error explained above. In this case, the welding axle 5 may belifted off from the surface of the workpiece by a distance which makesit possible to eject the part to be welded that is already held in thestud holding device 15 by further or again controlling the charging pin23. For this purpose, it is of course also possible to control the drive11 in such a way that the stud welding head is moved to a place whereejecting the part to be welded that is already present is possiblewithout danger.

[0068] Instead of positioning the welding axle at a distance X; from thesurface of the workpiece as shown in FIG. 2a, this process can bemodified such that the welding axle or the entire welding head is atfirst moved towards the surface of the workpiece by means of the drive11 until the stud holding device and the surface of the workpiececontact each other. At this point, however, the drive 11 is not stopped,but is moved further towards the workpiece until a predeterminedabsolute position of the welding head 5 is reached. In order to makesuch a movement possible after the stud holding device and the workpiecehave contacted each other, the drive 7 may be configured such and may becontrolled to enter such a condition that it is possible to axiallydisplace the welding axle 5 as a result of the further movement of thedrive 11. In other words, after the first contact between the studholding device and the workpiece, the drive 11 is moved further in sucha way that the entire welding head moves further in the axial direction.This movement is compensated by an axial countermovement of the weldingaxle 5 relative to the drive 7. In this way, too, it is possible toreach the position according to FIG. 2b. For this purpose, however, itis not necessary to detect a contact between the stud holding device andthe workpiece. Rather, the drive 11 for the stud welding head 3 may bemoved in such a way that in spite of tolerances occurring whenpositioning the workpiece 17 and when moving the arm 9 of the robot orthe entire welding head 3, an (absolute spatial) position is takensecurely, in which the face of the stud holding device 15 and thesurface of the workpiece 17 contact each other.

[0069]FIG. 3 shows different phases a to f of another method accordingto the invention.

[0070] In this method, in the first step, the welding axle or the entirestud welding head is again positioned such that there is a distanceX_(c) between a part 19 to be welded which is already held in the studholding device 15 and the surface of the workpiece. In the next step,preferably using the drive 7, the welding axle 5 is moved towards theworkpiece 17 until a contact is detected. The contact may be detected asexplained above. After the contact has been detected as shown in FIG.3b, the charging pin 23, which for the present has held the part 19 tobe welded in its position, is moved back. Then, another attempt is madeto move the welding axle 5 further towards the workpiece 17. As the part19 to be welded is not acted upon by the charging pin 23 any more, thisis possible if a part to be welded is really held in the stud holdingdevice 15. Otherwise, the face of the stud holding device would restupon the surface of the workpiece and would block such a furthermovement.

[0071] If the evaluation and control unit, which controls the entiresequence of movements, detects that such a further movement towards thesurface of the workpiece is possible, it is assumed that a part to bewelded is really held in the stud holding device 15. For this purpose,the evaluation and control unit can of course detect whether a movementof the welding axle 5 by a distance which is larger than a predeterminedthreshold value is possible.

[0072] When the attempt of a further movement towards the workpiece isstarted, the evaluation and control unit simultaneously detects thepotential existence of a “rigid mechanical contact” which exists if thestud holding device itself rests upon the workpiece. Again, this may beeffected by detecting the power consumption of the drive 7 or 11,depending on which of the two drives is employed for the movement.Preferably, however, the drive 7 for the welding axle will be chosen.

[0073] If a mechanical contact is detected without a movement of thewelding axle or the welding head towards the workpiece having beenpossible, the control unit 13 will assume that no part to be welded isheld in the stud holding device 15. In this case, the operation may bestopped and an error signal or an operator recall signal may begenerated, or the welding head may be lifted off from the workpiece by asufficient amount and an operation for the supply of a part 19 to bewelded may be triggered.

[0074] On this occasion, it is to be pointed out explicitly again that amechanical contact may also be detected by detecting that unusually highforces arise in the stud holding device or in the welding axle. For thispurpose, the evaluation and control unit can evaluate the signal of aforce or strain sensor.

[0075] When the evaluation and control unit 13 has detected the presenceof a part 19 to be welded in the stud holding device 15, the weldingaxle or the welding head can be lifted off from the workpiece and thedrive 25 can be controlled in such a way that the part to be welded ispushed to the front into its welding position again. This phase is shownin FIG. 3d.

[0076] Then, the welding head can be moved towards the workpiece 17again until the evaluation and control unit 13 detects a contact.Afterwards, one of the usual welding processes may be started. Forexample, for the stroke ignition method, the pre-current may be switchedon in the phase shown in FIG. 3e, so after lifting off the welding axleby the distance h according to FIG. 3f, the pre-current electric arc isstruck.

[0077] With the method shown in FIG. 3, it is also possible to decidewhether a stud having the right length is held in the stud holdingdevice if studs having different lengths are used: For this purpose,from the point of time when the stud contacts the workpiece and thecharging pin 23 is retracted or is enabled to freely move in the axialdirection again, the welding axle 5 is moved further towards theworkpiece until a contact between the stud holding device 15 and theworkpiece is detected (e.g. by detecting the power consumption of thedrive). The distance covered between the point at which the studcontacts the workpiece and the point at which the stud holding devicecontacts the workpiece corresponds to the projecting length d of thestud in the stud holding device. This projecting length is larger if alonger stud is present than if shorter studs are present—withoutchanging the front end position of the charging pin.

[0078] In this way, in the embodiments described above, the inventionmakes it possible to securely decide whether a part to be welded is heldin the stud holding device. Particularly in case of welding without asupporting foot, this will help to securely avoid that the stud holdingdevice itself is welded onto the workpiece if a part to be welded ismissing. In the usual welding methods, too, in which a supporting footis used, it is possible to avoid that an electric arc is struck betweenthe face of the stud holding device and the surface of the workpiece andthat the stud holding device is damaged thereby. Furthermore, it ispossible to draw the conclusion that the stud is positioned correctly inthe stud holding device or that a stud having the right length asdesired is present.

1. Stud welding method, particularly for stud welding without asupporting foot, for welding a part (19) to be welded which is held in astud holding device (15) of a stud welding head (3) to a workpiece (17),characterized by the following steps: a) moving the stud holding device(15), without a part (19) to be welded being held therein, towards thesurface of the workpiece at least until the face of the stud holdingdevice (15) contacts the surface of the workpiece; b) lifting the studholding device (15) off from the workpiece (17) until a predetermineddistance (X_(c)) between the face of the stud holding device (15) andthe surface of the workpiece is reached; c) supplying a part (19) to bewelded into the stud holding device (15) in such a way that the face ofthe part (19) to be welded projects from the face of the stud holdingdevice (15) by a predetermined length of projection (d); d) moving thestud holding device (15) towards the surface of the workpiece againuntil a contact between the face of the part (19) to be welded and thesurface of the workpiece is detected, and detecting the distance bywhich the stud holding device (15) or an element coupled therewith wasmoved from the position that was reached after step b) until the contactwas detected; e) comparing the distance detected with the distance thatwas determined according to step b), wherein the presence of a part (19)to be welded is assumed if the distance detected differs from thepredetermined distance (X_(c)) in that it is smaller at least by apredetermined amount; f) conducting the welding operation if thepresence of a part (19) to be welded was detected.
 2. Method accordingto claim 1, characterized in that it will be assumed that the part (19)to be welded is present in the stud holding device (15) in a positionthat is admissible for a welding operation if the difference between thepredetermined distance (X_(c)) and the distance detected is within apredetermined range.
 3. Method according to claim 1 or 2, characterizedin that the movement according to feature a) of claim 1 is only madeuntil a contact between the stud holding device (15) and the workpiece(17) is detected.
 4. Method according to one of claims 1 to 3,characterized in that the detection of the contact between the studholding device (15) and the surface of the workpiece and/or thedetection of the contact between the part (19) to be welded and thesurface of the workpiece is effected by detecting an electric ormechanical contact.
 5. Method according to one of the preceding claims,characterized in that at least all the movements of the stud holdingdevice (15) according to features b) and d) of claim 1 are effected by adrive (7) for the stud holding device (15) or for an element coupledtherewith, which drive is provided in the stud welding head (3) 6.Method according to one of the preceding claims, characterized in thatbefore carrying out the steps according to claim 1, the stud weldinghead (3) is positioned relative to the workpiece (17) by means of apositioning drive (11) in such a way that a distance (X_(c)) remainsbetween the face of the stud holding device (15) and the surface of theworkpiece, preferably at the desired place where the weld is to be made.7. Method according to claim 6, characterized in that the movement ofthe stud holding device (15) according to feature a) of claim 1 iseffected by means of the drive (7) for the stud holding device (15)provided in the stud welding head (3) or an element coupled therewith,too.
 8. Method according to one of claims 1 to 5, characterized in thatthe movement of the stud holding device (15) according to feature a) ofclaim 1 is effected by means of a positioning drive (11) for the studwelding head (3), with the movement of the welding head (3) beingeffected by open- or closed-loop control and into a position in which acontact between the face of the stud holding device (15) and the surfaceof the workpiece exists with sufficient certainty.
 9. Method accordingto claim 8, characterized in that the movement of the stud welding head(3) towards the workpiece (17) is effected in such a way that the studholding device (15) is moved from a starting position towards the studwelding head (3) and into the latter, with the drive (7) for the studholding device (15) or an element coupled therewith allowing such amovement, and that the movements of the stud holding device according tofeatures b) and d) of claim 1 are effected by the drive (7) for the studholding device (15) or an element coupled therewith.
 10. Stud weldingmethod, particularly for stud welding without a supporting foot, forwelding a part (19) to be welded which is held in a stud holding device(15) of a stud welding head (3) to a workpiece (17), characterized bythe following steps: a) moving the stud holding device (15), with a part(19) to be welded being held therein, towards the surface of theworkpiece at least until the face of the part (19) to be welded contactsthe surface of the workpiece, with the part (19) to be welded being heldin position in the stud holding device (15) in such a way that the faceof the part (19) to be welded projects from the face of the stud holdingdevice (15) by a predetermined length of projection (d); b) trying tomove the stud holding device (15) further towards the workpiece (17) anddetecting a possible further movement; c) assuming that a part (19) tobe welded is present if a further movement of the stud holding device(15) and thus an axial movement of the part (19) to be welded into thestud holding device (15) by at least a predetermined distance waspossible; d) lifting the stud holding device (15) and thus the face ofthe part (19) to be welded off from the surface of the workpiece by apredetermined distance; e) moving the part (19) to be welded into thestarting position in the stud holding device; f) conducting the weldingoperation if the presence of a part (19) to be welded was detected. 11.Method according to claim 10, characterized in that in an attempt tomove according to feature c) of claim 10, the power consumption of adrive (7; 11) for the stud holding device (15) or the welding head (3),or the force acting upon the stud holding device (15) or an elementcoupled therewith during the attempt to move the stud holding device isdetected and that, if a power consumption or a force acting whichexceeds a predetermined threshold is detected, the attempt to move isstopped.
 12. Method according to claim 10 or 11, characterized in thatbetween steps f) and g) of claim 10, the stud holding device (15) andthus the face of the part (19) to be welded are repositioned withrespect to their distance from the surface of the workpiece, which ispreferably effected by moving the stud holding device (15) towards theworkpiece (17) until a contact between the part (19) to be welded andthe workpiece (17) is detected, whereupon the stud holding device (15)is lifted off by a predetermined distance.
 13. Method according to oneof claims 1 to 9, characterized in that when supplying the part (19) tobe welded, its contact with the surface of the workpiece is monitored,and that, if a contact is detected, the presence of a part (19) to bewelded which is already held in the stud holding device before theprocess of supplying is already assumed during the process of supplyingand the process of supplying is preferably stopped.
 14. Method accordingto claim 13, characterized in that if necessary, the stud holding device(15) is moved into a position in which the part (19) to be welded whichis already present can be ejected by again starting the supply movementof the part (19) to be welded for bringing the same into its startingposition in the stud holding device (15).
 15. Stud welding device forcarrying out the method according to one of the preceding claims a)comprising a stud welding head (3) with a drive (7) for a stud holdingdevice (15) or an element coupled therewith, b) comprising means fordetecting an electric or mechanical contact between the stud holdingdevice (15) and a workpiece (17) or between a part (19) to be weldedwhich is held in the stud holding device (15) and the workpiece and c)an evaluation and control unit (13) which is connected with the drive(7) for the stud holding device (15) and the means for detecting anelectric or mechanical contact, characterized in that d) the evaluationunit (13) is configured such that it controls the drive (7) for the studholding device and the means for detecting an electric or mechanicalcontact in accordance with the process steps according to one of thepreceding claims.
 16. Device according to claim 15, characterized inthat the means for detecting an electric or mechanical contact comprisea unit for detecting the electric driving power of the drive (7) for thestud holding device or a parameter coupled therewith.
 17. Deviceaccording to claim 15 or 16, characterized in that a drive (11) formoving the entire welding head (3) is provided, which is preferablyadapted to be controlled by the evaluation and control unit (13). 18.Computer program with program code means for carrying out all the stepsof the method according to one of claims 1 to 14 if the program isexecuted in an evaluation and control unit (13) comprising amicroprocessor control.
 19. Computer program with program code meansaccording to claim 18 which are stored on a data carrier.